Phosphite type compounds, in both acid and salt forms, are known to be not only effective pesticides (fungicides), but also slow release fertilizers. For example, phosphorous acid is well known to possess both fungicidal and plant nutrition properties. Application to the soil or by foliar application, phosphite type products unfortunately become oxidized into phosphates and thereby lose their activity as a fungicide; thus becoming a traditional phosphate fertilizer.
Fungicidal activity is the most valuable characteristic of phosphite type products. Fungal diseases are an important concern when considering the potential for significant harm to plants. Because of this concern, it is most desirable to maintain at least a minimal pre-determined concentration of phosphite present in plant tissue to provide a level of continuous plant protection from fungal infestation.
Copper is known as a bactericide and fungicide as well as a plant nutrient. However, copper salts are known to cause plant phytotoxicity. As a rule, phytotoxicity of copper salts depends on the pH of the composition available to the plant. The lower the pH, the higher the risk of phytotoxicity. It is desirable if phosphorus and copper can be applied together to reduce the labor costs associated with separate applications.
It is not uncommon for plants to contract diseases caused by both fungi and bacteria. Application of a suitable amount of phosphorous acid with copper may simultaneously cure the diseases caused by both fungi and bacteria. However, mixing both phosphorous acid and copper together for field application is problematic. Mixing phosphorous acid with copper results in a product having a low pH which is phytotoxic to plants. However, if the product is adjusted to a higher pH, such as 6.5 or higher using potassium hydroxide, the product will form insoluble precipitates of copper hydroxide and copper phosphite.
The pH is an important factor that affects phosphite and the fungicidal effectiveness of other products when applied to plants. The pH range that provides the highest activity for a phosphite type fungicide, is between about 6.3-7.5. At the same time, pH of water in different areas of agricultural activities varies from slightly acidic (5.0-5.5) to slightly alkaline (8.5-9.0).
In experimental Agrochemistry, less damage occurs to plants by foliar application of copper containing compositions having a pH greater than 6.0. The inconvenience of this approach is the formation of water insoluble forms of copper compounds which become suspended in solution. Rather than this approach, it would be most desirable to provide a multifunctional composition containing copper at a pH above 7.0.